At the present time, the scanners are so popular that they are required to have more and more functions. The flatbed scanner used today has both transmissive mode and reflective mode, and so does the sheetfed scanner. When a reflective object such as a piece of paper needs to be scanned, the reflective mode must be adopted. The light source (hereafter called "reflecting light source") mounted at the same side of the charge coupled device is used to scan the reflective object. A reflected image signal is sensed by the photo sensor on the charge coupled device to be converted to digital data. When a transmissive object such as a transparency needs to be scanned, the transmissive mode, rather than the reflective mode, must be adopted. The light source (hereafter called "transmitting light source") mounted at the opposite side of the charge coupled device is used to scan the transmissive object. A transmitted image signal is sensed by the charge coupled device to be converted to digital data. If the scanning mode is selected incorrectly, the image gotten at last will be ambiguous. The trouble of using the scanner is that the users must know the type of the scanned object and switch among the reflective mode and the transmissive mode of the scanner by themselves. The users are often puzzled by such manual operation when a great number of non-arranged documents including both types of the scanned objects need to be scanned. Apparently, it is inconvenient for the users to switch the scanning mode of the conventional scanner.